Glaciers in the McMurdo Dry Valleys of Antarctica never rise far above the threshold of melting throughout summer.  They are rarely observed to be actively melting, yet runoff from these glaciers is the primary source of water to streams, lakes and associated ecosystems in the valleys, which are among the coldest and driest ecosystems on Earth.    The processes generating melt under marginal conditions are not well understood, and traditional melt modeling techniques are inadequate to explain the observed runoff from these glaciers.

Here we investigate the role of penetration of solar radiation into the ice and the subsequent drainage of subsurface melt from the ice in generating runoff from Dry Valley glaciers by successively adding these processes to a surface energy balance model.  The model is applied to three glacier sites using 13 years of hourly meteorological data.  Model results show that inclusion of both processes is necessary to accurately model ablation, ice density, and ice temperature on these glaciers.  Melt on the glacier surface is rare, but internal melting 5–15 cm below the ice surface is extensive and its drainage accounts for ~50% of all summer ablation. This is consistent with field observations of subsurface streams and formation of a weathering crust. We identify an annual cycle of weathering crust formation in summer and its removal during the 10 months of winter sublimation.

Due to the complexities of ice melt at air temperatures close to the melting temperature, these glaciers will respond differently to changes in climate than glaciers in warmer climates.  This will also apply to other glaciers at very high latitudes and elevations and many extraterrestrial glaciers such as those on Mars.